1. Field of the Invention
This invention relates to a method and circuit for reducing the influence of a bright image area in an endoscope image signal.
2. Related Art Statement
Recently, solid state imaging devices such as CCD's have come to be used as imaging means of various apparatuses.
However, the dynamic range of the so far existing imaging means is so narrow that it is difficult to produce a video signal which reflects the contrast of the natural field well.
For example, if the brightness of an object to be imaged is different from a proper exposure level of an imaging means, this part will become too dark an image or, the contrary, too bright on a displayed picture.
Recently, endoscopes are extensively utilized for medical and industrial uses. An endoscope provided with an imaging means has been developed. Such an endoscope usually has an illuminating means. Therefore, in a medical endoscope, when observing, for example, a mucous membrane of a living body, by the reflection of an illuminating light on the mucous membrane, a local bright part or bright image area, that is, a bright point will be produced. Also, in an industrial endoscope, when observing, for example, a metal surface, by the reflection of an illuminating light on the metal surface, a bright point or bright image area will be produced.
For example, FIG. 24(a) shows an example of a video signal in one horizontal period when a tubular object interior is imaged. If a lustrous object, such as a living body mucous membrane or metal surface, is present as an object to be imaged within a tube, a part (bright point) have a large reflected light amount will be locally generated. If such an object is imaged by an imaging device, the signal level of the above mentioned bright point will become so high that bright point signals l, l, . . . as are shown in FIG. 24(a) will be generated. Unless the luminance level of another view point is reduced until the observation is impossible, the luminance output of that bright point will become so large as to mostly saturate the imaging device.
The projecting bright point signals l, l, . . . will be integrated by a low pass filter (LPF) device of a video signal processing circuit and the widths (occupied areas) of the bright point signals l, l, . . . will apparently increase or will deteriorate a beam on a CRT and will cause the same phenomenon to occur.
Particularly, the bright point projecting out of a dark part is so large in the luminance difference from the periphery as to obstruct the observation and can be said to be an unnecessary high luminance part.
Therefore, the publication, for example, of Japanese Patent Application Laid Open No. 107674/1981 discloses a technique wherein a .gamma.-correcting amount of a which processes circuit processing signals can be switched so that a part which is very different in luminance may be observed even if it is present within a displayed picture.
However, in this technique, on the video signal shown in FIG. 24(a), the .gamma. characteristic on the entire picture is changed and the high luminance part is compressed to make video signal as is shown in FIG. 24(b). In such a case, the high luminance parts other than the bright point signals l, l, . . . will also be compressed, therefore necessary information will also be compressed and, as a result, the dynamic range of the necessary information part will be reduced. The .gamma. characteristic of the necessary information part will vary and therefore the fidelity of the reproduced image will deteriorate. As understood from FIG. 24(b), as the bright point signals (i.e., bright image area) l', l', . . . projecting out of the central part (dark part) of the picture are still displayed at a high luminance, the observation of the part desired to be observed will be obstructed.
The publication of Japanese Patent Application Laid Open No. 272768/1987 discloses a distortion removing circuit wherein a video signal is input into an LPF and is compared with an original signal not transmitted through the LPF and the undershoot or the like of the original signal is clipped with the difference signal.
This prior art example is to prevent the unnecessary undershoot in the video signal from causing a misoperation at the time of a synchronizing separation or the like. In this prior art example, an original signal including the undershoot shown in FIG. 25(a) is passed through a low pass filter to obtain a video signal as is shown in FIG. 25(b) and, in case the difference between this video signal and the original signal shown in FIG. 25(a) is above a predetermined level, the original signal will be clipped with a level limiter to obtain the signal shown in FIG. 25(c).
This prior art example is to prevent the undershoot in the video signal from being misconceived as a synchronizing signal which is a signal projecting on the black level side in the video image. Therefore, the above mentioned prior art example is to remove a distortion projecting on the black level side. On the other hand, a bright point projects on the white level side and can not be removed in the above mentioned prior art example.
In case the imaging device is a solid state imaging device of a single plate color imaging system using a color mosaic filter or the like, by the difference in the transmissivity of the respective color filters for color separation, the saturated light amounts of the corresponding imaging pixels will be different and therefore a psuedo-color will be generated at the time of a high luminance incidence.
A pseudo-color will be generated also in the part having no pixel correlation with the components (for example, red (R), green (G), blue (B), magenta (Mg), G, cyanine (Cy) and yellow (Ye)) of a color filter separating a white color, for example, in the edge part of the video image. In such a case, the tone of the generated pseudo-color will be different depending on what color filter part among the components lacks the incidence upon the edge part.
Therefore, as prior art examples improving the above mentioned defect, there are Japanese Patent Applications Laid Open Nos. 114823/1976 and 17291/1982 and Japanese Utility Model Application Laid Open No. 34793/1986. The first and second prior art examples are to detect a high luminance part and depress a color signal on that part. The third prior art example is to somewhat widen a detected high luminance part so that the lag of the pseudo-color generating range by the lag element of the video circuit system and the edge part pseudo-color may have a depressing width margin.
The disadvantages of the above mentioned prior art examples shall be explained in the following with reference to FIG. 26.
FIG. 26 shows an example of a video signal in one horizontal scanning period. This video signal is a waveform in which spike-like bright point parts are overlapped in a low range component in which the luminance level varies as inclined.
For the above mentioned reasons, a pseudo-color (high luminance coloring) will be generated in the high luminance part, for example, a part that exceeds Vref in FIG. 26 and also in the part in which the luminance variation is violent, that is, in the spike-like bright point parts.
On the other hand, the first to third prior art examples are to merely detect a high luminance part and depress the chroma saturation and therefore can reduce the pseudo-color of the above mentioned former but have no function of reducing the latter, that is, the bright points (shown by l.sub.i, l.sub.2, . . . in FIG. 26) having levels below Vref.